Suppose I want to cover a roof with corrugated steel sheets. Imagine a sheet laying on the roof. It will have "tops" - zones that are further from the roof surface and "bottoms" - zones that are closer to the roof surface (the line is the sheet cross-section, the roof is below the line, the outdoors is above the line):

I need to decide where I run the screws that will hold the sheet.

In case it was roofing slate I'd definitely run them through the "tops" ("B" on the drawing) because when it rains water will run along the "bottom" and into any hole it finds there.

However seems like the typical approach with corrugated steel is to run the screws through the "bottom" ("A" on the drawing) which puzzles me a lot.

3 Answers
3

If you choose B you will dimple the steel, ruining the look while creating a penetration point for water because expansion and contraction of the metal due to heating and cooling will create an open access point for water to enter, and a big opening behind it to receive it.

A, however, will give you a tight seal against the wood, wood-steel-screw in a nice tight sandwich - resisting expansion/contraction gaps and thus protecting much more effectively against water penetration while preserving the look of the corrugated steel

When you install slate, you are installing shingles, and you're overlaying them in a pattern where the top of the shingle is against the underlying surface (wood, tar paper) and the bottom of the shingle is over the top of the next course down. With slate as with asphalt, you're securing the top of the shingle tight to the wood underneath, and then covering those holes with the next layer. Corrugated sheeting works completely differently. So forget shingles - learn this product.
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The Evil GreeboJun 28 '12 at 13:22

My link says "Horizontal Wood Strips are used on every cross support". And page 2 of the following guide shows this in a picture. eplastics.com/pdf/… But this is fiberglass and perhaps metal is different.
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Philip NgaiJun 29 '12 at 7:24

wiggle molding will allow you to screw on the high points of the metal without making a dimple in the sheet. It costs a little more but is worth it if you are unsure of your screw points and want it to last longer.